In high-speed, high-quality electrophotographic printing applications, it may be desirable to get high gloss on the pictorial areas of an image but not on the text areas (i.e., differential gloss). As described in U.S. Pat. No. 5,234,783, issued to Ng, herein incorporated in its entirety by reference, this may be accomplished by selectively putting a transparent toner overcoat on the pictorial area. One example described in the Ng patent makes use of a lower viscosity toner so that there can be a higher gloss in the pictorial areas.
However, with high speed and high quality printing, there still can be disadvantages from the viewpoint of achieving higher gloss with heated roller fusing. For example, too much total toner coverage on the media may stress the fusing subsystem. Moreover, at the higher temperatures required to fuse a transparent toner overcoat along with the color toner lay-down, roller reliability as well as artifacts from the fuser roller oiler may become problematic. Additionally, there may also be problems relating to image relief differences between toner-covered areas versus adjacent areas without the transparent toner overcoat.
Other conventional systems to increase image gloss include an ultraviolet (UV) curable overcoat that may be applied over the total image or over, for example, only pictorial portions of the image. The UV curable overcoat may be applied by a conventional commercial printing coater or by ink jet printing, wherein a specific area may be coated selectively. However, with UV curable inks, even though image protection may be achieved over a wide variety of media, only certain types of coated media can benefit from the UV coating to lower differential gloss. In some cases, with uncoated matte media for example, differential glass can get worse with UV coating. Moreover, because most UV curable ink layers are a few microns thick, image relief may be quite visible on the dry electrophotographic prints.
As can be seen, there is a need for improved control of differential gloss on a wide variety of media substrates while minimizing image relief that may result from certain conventional differential gloss control methods.